1. Field of the Invention
This invention relates to charged particle mirrors and, more specifically, to an ion mirror used in time-of-flight mass spectroscopy. The invention provides a continuous voltage gradient that allows more precise and efficient sample analysis.
2. Description of the Related Art
In time-of-flight-spectroscopy, ions are formed in a short source region in the presence of an electric field that accelerates the ions into a longer, field-free drift region. Ideally, the electric field imparts the same kinetic energy (KE) to the ions equally so that they will have different velocities, which depend on their mass. The time (t) required for the ions to traverse the drift region depends on the mass of the ion. The time axis in a time-of-flight mass spectrometer reflects not only the mass but the initial energy distributions of the ions (temporally, spatially, and kinetically), their fate during acceleration, and properties of the recording system. Due to a distribution of internal energies, two ions of the same mass can be accelerated from the same location but have different velocities (kinetic energies). When this occurs, a distribution in arrival times at the detector is recorded causing a loss of resolution. A further loss of resolution is caused by ions accelerated from different locations.
The resolution may be improved by applying high accelerating voltages, thus minimizing the contribution from different ion energies or by using an ion mirror, a "reflectron", as suggested by Mamyrin et al. in 1973, to correct for the temporal effects of initial kinetic energy distributions. The reflectron, located at the end of the flight tube, consists of a series of rings and/or grids with voltages that increase (linearly in the simplest case) up to a value slightly greater than the voltage at the ion source. The ions penetrate the reflectron until they reach zero kinetic energy, turn around, and are reaccelerated back through the reflectron, exiting with energies identical to their incoming energy but with velocities in the opposite direction. Ions with more energy penetrate the reflection deeper and will have longer flight paths than those with less energy. These higher energy ions can be made to arrive at the detector at very nearly the same time as less energetic ions, thereby compensating for the energy spread. Unfortunately, the ions experience a piecewise-linear electric field gradient due to the discrete nature of voltages on each ring. Ions near the inner perimeter can be lost and external electric fields can affect the remaining ions. Furthermore, this "series of rings" is bulky and costly to manufacture. The rings present a large surface area to the vacuum system, which requires additional pumping capacity to handle the potentially large initial water vapor and desorbed gas load.
What is needed is a controlled gradient device that is capable of generating a continuous electric field gradient to maximize useful signal from the ion sample. It would be further beneficial if the controlled gradient device were self-shielding to minimize the effect of any external electric fields on the ion sample.